<header>
    <h1>在线加密/解密工具</h1>
    <nav>
        <a href="/" class="{% if request.path == '/' %}active{% endif %}">首页</a>
        <a href="/encrypt" class="{% if request.path == '/encrypt' %}active{% endif %}">加密</a>
        <a href="/decrypt" class="{% if request.path == '/decrypt' %}active{% endif %}">解密</a>
    </nav>
</header>